Ring laser gyros use ring lasers that are formed in solid blocks of material that, over a wide temperature range, are resistant to expansion.
Several proprietary materials, such as the material known by the Trademark, Zerodur, exhibit excellent dimensional stability over wide temperature ranges. The stability is found to be related to the presence of lithium oxide in the material. The mentioned Zerodur, for example, has approximately 2.3% lithium oxide, and to prevent the altering of the temperature stability of the material, the ions of lithium need to be prevented from migrating.
It is usual to bore a laser path in a block of the temperature stable material and to place mirrors at the corners of the path. The mirrors are usually frequency sensitive and very narrow band mirrors so that their reflectivity in the desired laser frequency band is almost 100%. To achieve that reflectivity, the mirrors are coated with multiple layers of differing materials. Those differing materials cannot be contaminated without changing their indices of refraction and altering the characteristics of the mirrors.
Frequently the laser gas within the conduit is excited by placing voltages between spaced-apart anodes and a cathode which are attached to or embedded in the laser block. The portion of the laser path between the cathode and the anodes is designated the gain bore.
Usually the anodes are grounded. Other parts of the support structure for the laser gyro are metallic, and they are also grounded. When these grounded parts contact the last block, such parts become positive means for producing an electric field, and a voltage is placed between such means for producing an electric field and the cathode surfaces which are in contact with the last block.
The voltages between any positive means for producing an electric field and the cathode create an electric field in the laser block and in the mirrors, tending to force lithium ions away from such electrodes and toward the cathode, sweeping the lithium ions from the laser block and altering its temperature stability. Further, the ions may be swept into a mirror coating, thereby altering the indices of refraction of the films of the coating.
Another detrimental symptom occurs at the cathode seal; The lithium ions, arriving at the cathode, are neutralized by electrons as soon as they reach the metal. Hence lithium atoms accumulate at the cathode seal surface and endanger the integrity of the seal.
Suppression of ion flow into the mirrors has been achieved by surrounding the mirrors with insulating collars which do not contain lithium or other migrating ions.